Photoelectrochemical cell for carbon dioxide conversion

ABSTRACT

the present disclosure relates to photoelectrochemical cells and methods for using such for reduction of carbon dioxide and oxidation of water. In one aspect, the disclosure provides a method of electrochemically reducing carbon dioxide in an electrochemical cell, comprising contacting the carbon dioxide with at least one transition metal dichalcogenide in the electrochemical cell and at least one helper catalyst and applying a potential to the electrochemical cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of U.S. ProvisionalPatent Application No. 62/436,870, filed Dec. 20, 2017, which is herebyincorporated herein by reference in its entirety.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

This disclosure relates generally to photoelectrochemical cells. Moreparticularly, the present disclosure relates to photoelectrochemicalcells and methods for using such for reduction of carbon dioxide andoxidation of water.

Technical Background

In 2013, the global concentration of carbon dioxide in the atmospherereached 400 parts per million (ppm) for the first time in recordedhistory. Such levels will cause radical and largely unpredictablechanges in the environment. Recent efforts have shown that CO₂ can beconverted by electrochemical reduction processes driven by renewableenergy sources into energy-rich fuels (e.g., syngas, methanol), offeringan efficient path for both CO₂ remediation and an alternative energysource.

The chemical inertness of CO₂, however, renders most conversionprocesses highly inefficient. Current catalysts are plagued by weakbinding interactions between the reaction intermediates and the catalyst(giving rise to high overpotentials), or by slow electron transferkinetics (giving rise to low exchange current densities).

A photoelectrochemical cell capable of carbon dioxide reduction andwater oxidation may generate, e.g., CO, O₂, and/or H₂ by irradiating aphotovoltaic cell with light, generating spatially separated electronhole pairs. The generated pairs may be captured by catalysts capable ofreducing carbon dioxide or oxidizing water. Current attempts at suchsystems have been limited by expensive light-absorbing materials and/orcatalysts, and by the requirement for strongly acidic or basic reactionmedia, which are corrosive and difficult to manage or a large scale.

Accordingly, there remains a need for photoelectrochemical systemscapable of reducing CO₂ and/or oxidizing water using robust, relativelyinexpensive catalysts and manageable reaction media.

SUMMARY OF THE DISCLOSURE

One aspect of the disclosure is a method of electrochemically reducingcarbon dioxide and oxidizing water in an electrochemical device, themethod comprising providing an electrochemical device, the deviceincluding a first and second compartment and at least one photovoltaiccell, wherein

the first compartment includes

-   -   a cathode in electrical contact with at least one transition        metal dichalcogenide,    -   a first electrolyte, and    -   carbon dioxide, carbonic acid, or a carbonic acid salt;

the second compartment includes

-   -   an anode in electrical contact with at least one water oxidizing        catalyst,    -   a second electrolyte, and    -   water;

the at least one photovoltaic cell is in electrical contact with theanode and the cathode; and

the first compartment is in ionic contact with the second compartment;and

exposing the photovoltaic cell to light irradiation sufficient to createa potential difference between the anode and the cathode sufficient toreduce carbon dioxide at the cathode and to oxidize water at thecathode.

Another aspect of the disclosure is an electrochemical device having afirst and second compartment and at least one photovoltaic cell, wherein

the first compartment includes

-   -   a cathode in electrical contact with at least one transition        metal dichalcogenide,    -   a first electrolyte, and    -   carbon dioxide, carbonic acid, or a carbonic acid salt;

the second compartment includes

-   -   an anode in electrical contact with at least one water oxidizing        catalyst,    -   a second electrolyte, and    -   water;

the at least one photovoltaic cell is in electrical contact with theanode and the cathode; and

the first compartment is in ionic contact with the second compartment.

Another aspect of the disclosure is a method of electrochemicallyreducing carbon dioxide in an electrochemical cell, comprisingcontacting the carbon dioxide with at least one transition metaldichalcogenide in the electrochemical cell and at least one helpercatalyst and applying a potential to the electrochemical cell, whereinthe at least one transition metal dichalcogenide is WSe₂ or WS₂.

Another aspect of the disclosure is a method of electrochemicallyreducing carbon dioxide comprising

providing an electrochemical cell having

-   -   a cathode in contact with at least one transition metal        dichalcogenide, and    -   an electrolyte comprising at least one helper catalyst in        contact with the cathode and the at least one transition metal        dichalcogenide,    -   wherein the at least one transition metal dichalcogenide is WSe₂        or WS₂;

providing carbon dioxide to the electrochemical cell; and

applying a potential to the electrochemical cell.

Another aspect of the disclosure is an electrochemical cell having acathode in contact with at least one transition metal dichalcogenide anda first electrolyte comprising at least one helper catalyst, wherein theat least one transition metal dichalcogenide is WSe₂ or WS₂.

Another aspect of the disclosure is an electrochemical cell having acathode in contact with at least one transition metal dichalcogenide anda first electrolyte comprising at least one helper catalyst, wherein theat least one transition metal dichalcogenide is WSe₂ or WS₂.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an electrochemical device100 comprising a first compartment 120 including at least one transitionmetal dichalcogenide 122 disposed on a cathode 121, which cathode isdisposed on at least one photovoltaic cell 130. Device 100 alsocomprises a second compartment 140 including at least one wateroxidizing catalyst 142 disposed on an anode 141, which anode is disposedon cell 130. Compartments 120 and 140 include a first electrolyte 123and a second electrolyte 143, respectively, and are in ionic contactthrough an ion-conductive membrane 150.

FIG. 2 is a schematic cross-sectional view of an electrochemical device200 comprising a first compartment 220 including at least one transitionmetal dichalcogenide 222 disposed on a cathode 221. Device 200 alsocomprises a second compartment 240 including at least one wateroxidizing catalyst 242 disposed on an anode 241. Compartments 220 and240 include a first electrolyte 223 and a second electrolyte 243,respectively, and a separated by, and in ionic contact through, anion-conductive membrane 250.

FIG. 3 is an optical image of a crystalline WSe₂ structure grown by thechemical vapor transport technique of Example 1, described in moredetail below. The scale bar is 10 μm.

FIG. 4 is an image of transition metal dichalcogenide nanoflakesprepared according to Example 2, described in more detail below.

FIG. 5 is a set of plots showing the normal size distributions of thetransition metal dichalcogenide nanoflakes synthesized according toExample 2, described in more detail in Example 3, below.

FIG. 6 is a set of Raman spectra of the transition metal dichalcogenidenanoflakes synthesized according to Example 2, described in more detailin Example 3, below.

FIG. 7 is a scanning electron microscopy (SEM) image of WSe₂ nanoflakes,described in more detail in Example 3, below.

FIG. 8 is a schematic view of the two-compartment three-electrodeelectrochemical cell of Example 4, described in more detail below.

FIG. 9 is a set of cyclic voltammetry (CV) curves for WSe₂ nanoflakes,bulk MoS₂, Ag nanoparticles, and bulk Ag in a CO₂ environment, asdescribed in more detail in Example 4, below. Inset highlights thecurrent densities under low overpotentials.

FIG. 10 is a set of CV curves for MoS₂, WS₂, MoSe₂, and WSe₂ nanoflakesin a CO₂ environment, as described in more detail in Example 4, below.

FIG. 11 is a set of two chronoamperometry (CA) experiments carried outfor an hour at different applied potentials, as described in more detailin Example 4, below. FIG. 11 (A) shows the results at −0.164 V and−0.264 V, and (B) shows the results at −0.764 V, −0.564 V, and −0.364 V.

FIG. 12 is a plot of the current density of CO₂ reduction by the WSe₂catalyst obtained through CA and pH of the electrolyte as a function ofthe water volume fraction of the electrolyte, as described in moredetail in Example 5, below.

FIG. 13 is a set of CV curves for (A) Ag nanoparticles, (B) bulk MoS₂,and (C) WSe₂ nanoflakes at different scan rates. Curves were obtained in0.5 M H₂SO₄ by sweeping from 0 to +0.3V vs RHE, as described in moredetail in Example 6, below.

FIG. 14 is a set of plots showing the current density of the CVexperiments shown in FIG. 13 at +0.2 V vs RHE as a function of scanrate. The slope of the linear fit provides the double layer capacitancefor each material, as described in more detail in Example 6, below.

FIG. 15 is a plot of the CO formation turnover frequency (TOF) of WSe₂nanoflakes, bulk MoS₂, and Ag nanoparticles at overpotentials of 54 to650 mV, as described in more detail in Example 6, below.

FIG. 16 is a calibration curve for CO production analysis by the gaschromatography (GC) setup of Example 7, described in more detail below.

FIG. 17 is a calibration curve for H₂ production analysis by the GCsetup of Example 7, described in more detail below.

FIG. 18 is a differential electrochemistry mass spectrometry (DEMS)spectrum of the product of the ¹³CO₂ reduction experiment described inmore detail in Example 7, below.

FIG. 19 is a plot of the Faradaic efficiency (FE) of CO and H₂production by WSe₂ nanoflakes as a function of applied potential, asdescribed in more detail in Example 8, below.

FIG. 20 is a plot of the FE of CO and H₂ production by MoS₂ nanoflakesas a function of applied potential, as described in more detail inExample 8, below.

FIG. 21 is a plot of the FE of CO and H₂ production by MoSe₂ nanoflakesas a function of applied potential, as described in more detail inExample 8, below.

FIG. 22 is a plot of the FE of CO and H₂ production by WS₂ nanoflakes asa function of applied potential, as described in more detail in Example8, below.

FIG. 23 is a plot of the performance (the product of the current densityand faradaic efficiency) of several catalytic materials as a function ofoverpotential, as described in more detail in Example 8, below.

FIG. 24 is a plot of the current density of WSe₂ nanoflakes as afunction of time over a 27-hour stability test, as described in moredetail in Example 9, below.

FIG. 25 is a schematic cross-sectional view of the electrochemicaldevice of Example 10, as described in more detail below.

FIG. 26 is a schematic representation of the (A) transient and (B)steady state operation regimes of the electrochemical device of Example10, as described in more detail in Example 11, below.

FIG. 27 is a plot of the rate of CO and H₂ formation of theelectrochemical device of Example 10, as described in more detail inExample 12, below.

FIG. 28 is a set of optical images of the indium tin oxide (ITO) layerof the photovoltaic (PV) cell of the electrochemical device of Example10 (A) before and (B) after 5 hours of continuous operation, asdescribed in more detail in Example 13, below. (C) shows a selectedregion of the corrosion in more detail. Scale bars are 250 μm.

FIG. 29 is a set of plots of (A) the rate of product formation of theelectrochemical device of Example 10 under different illumination levelsand (B) the solar-to-fuel efficiency (SFE) of the device, as describedin more detail in Example 15, below.

FIG. 30 is a plot of the SFE of the device of Example 10 as a functionof time, as described in more detail in Example 15, below.

FIG. 31 is a representative electrochemical impedance spectroscopy (EIS)spectrum for WSe₂ nanoflakes, bulk MoS₂, and Ag nanoparticles at variousoverpotentials, as described in more detail in Example 17, below. Thesmallest curve is for WS₂ NFs and the largest is for Ag NPs.

FIG. 32 is a plot of the work functions of various materials calculatedusing ultraviolet photoelectron spectroscopy, as described in moredetail in Example 18, below.

FIG. 33 is a set of images and corresponding intensity profiles of aWSe₂ nanoflake (A-B) before and (C-D) after a 27-hour CA experiment, asdescribed in more detail in Example 19, below. Scale bars are 2 nm.

FIG. 34 is a set of representative X-ray photoelectron spectroscopy(XPS) spectra of a WSe₂ nanoflake (A-B) before and (C-D) after a 27-hourCA experiment, as described in more detail in Example 20, below.

FIG. 35 is a set of plots of the partial density of states of the d band(spin up) of (A-C) the surface bare metal edge atom (Mo and W) of theMoS₂, MoS₂, and WSe₂ nanoflakes, respectively, and (D) the surface Agatom of bulk Ag(111), as described in more detail in Example 21, below.

FIG. 36 is a set of plots of (A) the calculated partial density ofstates of the d band (spin up) of the surface Ag atom of Ag₅₅ and (B)the surface bare metal edge atom (W) of WSe₂ nanoflakes, as described inmore detail in Example 21, below.

FIG. 37 is a set of the calculated free energy diagrams for CO₂electroreduction to CO an Ag(111), Ag55 nanoparticles, and MoS₂, WS₂,MoSe₂, and MoS₂ nanoflakes at 0 V vs RHE, as described in more detail inExample 21, below. The traces, top-to-bottom, are for Ag (111), Ag₅₅,MoSe₂, MoS₂, WSe₂ and WS₂, respectively.

FIG. 38 is a plot of the theoretical work functions calculated fortransition metal dichalcogenide monolayers, as described in more detailin Example 21, below.

DETAILED DESCRIPTION

The particulars shown herein are by way of example and for purposes ofillustrative discussion of the preferred embodiments of the presentdisclosure only and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of various embodiments of thedisclosure. In this regard, no attempt is made to show structuraldetails of the devices and methods described herein more detail than isnecessary for the fundamental understanding of the devices and methodsdescribed herein, the description taken with the drawings and/orexamples making apparent to those skilled in the art how the severalforms of the devices and methods described herein may be embodied inpractice. Thus, before the disclosed processes and devices aredescribed, it is to be understood that the aspects described herein arenot limited to specific embodiments, apparati, or configurations, and assuch can, of course, vary. It is also to be understood that theterminology used herein is for the purpose of describing particularaspects only and, unless specifically defined herein, is not intended tobe limiting.

The terms “a,” “an,” “the” and similar referents used in the context ofdescribing the methods and devices of the disclosure (especially in thecontext of the following claims) are to be construed to cover both thesingular and the plural, unless otherwise indicated herein or clearlycontradicted by context. Recitation of ranges of values herein is merelyintended to serve as a shorthand method of referring individually toeach separate value falling within the range. Unless otherwise indicatedherein, each individual value is incorporated into the specification asif it were individually recited herein. Ranges can be expressed hereinas from “about” one particular value, and/or to “about” anotherparticular value. When such a range is expressed, another aspectincludes from the one particular value and/or to the other particularvalue. Similarly, when values are expressed as approximations, by use ofthe antecedent “about,” it will be understood that the particular valueforms another aspect. It will be further understood that the endpointsof each of the ranges are significant both in relation to the otherendpoint, and independently of the other endpoint.

All methods described herein can be performed in any suitable order ofsteps unless otherwise indicated herein or otherwise clearlycontradicted by context. The use of any and all examples, or exemplarylanguage (e.g., “such as”) provided herein is intended merely to betterilluminate the methods and devices of the disclosure and does not pose alimitation on the scope of the disclosure otherwise claimed. No languagein the specification should be construed as indicating any non-claimedelement essential to the practice of the methods and devices of thedisclosure.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words ‘comprise’, ‘comprising’, and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to”. Words using the singular or pluralnumber also include the plural and singular number, respectively.Additionally, the words “herein,” “above,” and “below” and words ofsimilar import, when used in this application, shall refer to thisapplication as a whole and not to any particular portions of theapplication.

As will be understood by one of ordinary skill in the art, eachembodiment disclosed herein can comprise, consist essentially of orconsist of its particular stated element, step, ingredient or component.As used herein, the transition term “comprise” or “comprises” meansincludes, but is not limited to, and allows for the inclusion ofunspecified elements, steps, ingredients, or components, even in majoramounts. The transitional phrase “consisting of” excludes any element,step, ingredient or component not specified. The transition phrase“consisting essentially of” limits the scope of the embodiment to thespecified elements, steps, ingredients or components and to those thatdo not materially affect the embodiment.

Unless otherwise indicated, all numbers expressing quantities ofingredients, properties such as molecular weight, reaction conditions,and so forth used in the specification and claims are to be understoodas being modified in all instances by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forth inthe specification and attached claims are approximations that may varydepending upon the desired properties sought to be obtained by methodsand devices of the present disclosure. At the very least, and not as anattempt to limit the application of the doctrine of equivalents to thescope of the claims, each numerical parameter should at least beconstrued in light of the number of reported significant digits and byapplying ordinary rounding techniques. When further clarity is required,the term “about” has the meaning reasonably ascribed to it by a personskilled in the art when used in conjunction with a stated numericalvalue or range, i.e. denoting somewhat more or somewhat less than thestated value or range, to within a range of ±20% of the stated value;±19% of the stated value; ±18% of the stated value; ±17% of the statedvalue; ±16% of the stated value; ±15% of the stated value; ±14% of thestated value; ±13% of the stated value; ±12% of the stated value; ±11%of the stated value; ±10% of the stated value; ±9% of the stated value;±8% of the stated value; ±7% of the stated value; ±6% of the statedvalue; ±5% of the stated value; ±4% of the stated value; ±3% of thestated value; ±2% of the stated value; or ±1% of the stated value.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the disclosure are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical value, however, inherently contains certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements.

Groupings of alternative elements or embodiments of the disclosuredisclosed herein are not to be construed as limitations. Each groupmember may be referred to and claimed individually or in any combinationwith other members of the group or other elements found herein. It isanticipated that one or more members of a group may be included in, ordeleted from, a group for reasons of convenience and/or patentability.When any such inclusion or deletion occurs, the specification is deemedto contain the group as modified thus fulfilling the written descriptionof all Markush groups used in the appended claims.

Some embodiments of this disclosure are described herein, including thebest mode known to the inventors for carrying out the devices andmethods of the disclosure. Of course, variations on these describedembodiments will become apparent to those of ordinary skill in the artupon reading the foregoing description. Skilled artisans will employsuch variations as appropriate, and the it is intended for the devicesand methods of the disclosure to be practiced otherwise thanspecifically described herein. Accordingly, this disclosure includes allmodifications and equivalents of the subject matter recited in theclaims appended hereto as permitted by applicable law. Moreover, anycombination of the above-described elements in all possible variationsthereof is encompassed by the disclosure unless otherwise indicatedherein or otherwise clearly contradicted by context.

Furthermore, numerous references have been made to patents and printedpublications throughout this specification. Each of the cited referencesand printed publications are individually incorporated herein byreference in their entirety.

In closing, it is to be understood that the embodiments of thedisclosure disclosed herein are illustrative of the principles of themethods and devices of the present disclosure. Other modifications thatmay be employed are within the scope of the disclosure. Thus, by way ofexample, but not of limitation, alternative configurations of themethods and devices of the present disclosure may be utilized inaccordance with the teachings herein. Accordingly, the methods anddevices of the present disclosure are not limited to that precisely asshown and described.

In various aspects and embodiments, the disclosure relates to theelectrochemical or photoelectrochemical reduction of carbon dioxide and,optionally, the oxidation of water in a device including a cathodecomprising at least one transition metal dichalcogenide and, optionally,an anode comprising a metal and a photovoltaic cell. The disclosuredemonstrates such methods and devices to efficiently reduce carbondioxide, and to involve robust, relatively inexpensive catalysts andmanageable reaction media.

One aspect of the disclosure is a method of electrochemically reducingcarbon dioxide and oxidizing water. The method includes providing anelectrochemical cell comprising a first compartment including a cathodein contact with at least one transition metal dichalcogenide, carbondioxide, and a first electrolyte, the first compartment being in ioniccontact with a second compartment including an anode in contact with atleast one water oxidizing catalyst, water, and a second electrolyte, anda photovoltaic cell in electrical contact with the anode and thecathode. The method also includes exposing the photovoltaic cell tolight irradiation, for example, sufficient to create a potentialdifference between the anode and the cathode sufficient to reduce carbondioxide at the cathode and to oxidize water at the cathode. The lightirradiation can be, for example, at an average intensity of at least 0.5sun, for example at least 0.75 sun or at least 0.9 sun. For example, thelight irradiation can be, in various example embodiments of the devicesand methods as described herein, in the range of 0.5 sun to 3 sun, or0.75 sun to 3 sun, or 0.9 sun to 3 sun, or 0.5 sun to 2 sun, or 0.75 sunto 2 sun, or 0.9 sun to 2 sun.

In the methods and devices of the disclosure, the cathode of the firstcompartment is in contact with at least one transition metaldichalcogenide. In some embodiments of the methods and devices asotherwise described herein, the transition metal dichalcogenide is,e.g., TiX₂, VX₂, CrX₂, ZrX₂, NbX₂, MoX₂, HfX₂, WX₂, TaX₂, TcX₂, or ReX₂,wherein X is independently S, Se, or Te. In some embodiments, thetransition metal dichalcogenide is TiX₂, MoX₂, or WX₂, wherein X isindependently S, Se, or Te. In some embodiments, the transition metaldichalcogenide is TiS₂, TiSe₂, MoS₂, MoSe₂, WS₂, or WSe₂. In oneembodiment, the transition metal dichalcogenide is MoS₂ or WS₂. Inanother embodiment, the transition metal dichalcogenide is MoSe₂ orWSe₂. In yet another embodiment, the transition metal dichalcogenide isMoSe₂ or WSe₂. In one example, the transition metal dichalcogenide isMoS₂. In another example, the transition metal dichalcogenide is WSe₂.

The at least one transition metal dichalcogenide can be provided in avariety of forms, for example, as a bulk material, in nanostructureform, as a collection of particles, and/or as a collection of supportedparticles. As the person of ordinary skill in the art will appreciate,the transition metal dichalcogenide in bulk form may have a layeredstructure as is typical for such compounds. The transition metaldichalcogenide may have a nanostructure morphology, including but notlimited to monolayers, nanotubes, nanoparticles, nanoflakes (e.g.,multilayer nanoflakes), nanosheets, nanoribbons, nanoporous solids etc.As used herein, the term “nanostructure” refers to a material with adimension (e.g., of a pore, a thickness, a diameter, as appropriate forthe structure) in the nanometer range (i.e., greater than 1 nm and lessthan 1 μm). In some embodiments, the transition metal dichalcogenide isa layer-stacked bulk transition metal dichalcogenide with metalatom-terminated edges (e.g., MoS₂ with molybdenum-terminated edges). Inother embodiments, transition metal dichalcogenide nanoparticles (e.g.,MoS₂ nanoparticles) may be used in the devices and methods of thedisclosure. In other embodiments, transition metal dichalcogenidenanoflakes (e.g., nanoflakes of MoS₂) may be used in the devices andmethods of the disclosure. Nanoflakes can be made, for example, vialiquid exfoliation, as described in Coleman, J. N. et al.,“Two-dimensional nanosheets produced by liquid exfoliation of layeredmaterials.” Science 331, 568-71 (2011) and Yasaei, P. et al.,“High-Quality Black Phosphorus Atomic Layers by Liquid-PhaseExfoliation.” Adv. Mater. (2015) (doi:10.1002/adma.201405150), each ofwhich is hereby incorporated herein by reference in its entirety. Inother embodiments, transition metal dichalcogenide nanoribbons (e.g.,nanoribbons of MoS₂) may be used in the devices and methods of thedisclosure. In other embodiments, transition metal dichalcogenidenanosheets (e.g., nanosheets of MoS₂) may be used in the devices andmethods of the disclosure. The person of ordinary skill in the art canselect the appropriate morphology for a particular device.

In certain embodiments of the methods and devices as otherwise describedherein, the transition metal dichalcogenide nanostructures (e.g.,nanoflakes, nanoparticles, nanoribbons, etc.) have an average sizebetween about 1 nm and 1000 nm. The relevant size for a nanoparticle isits largest diameter. The relevant size for a nanoflake is its largestwidth along its major surface. The relevant size for a nanoribbon is itswidth across the ribbon. The relevant size for a nanosheet is itsthickness. In some embodiments, the transition metal dichalcogenidenanostructures have an average size between from about 1 nm to about 400nm, or about 1 nm to about 350 nm, or about 1 nm to about 300 nm, orabout 1 nm to about 250 nm, or about 1 nm to about 200 nm, or about 1 nmto about 150 nm, or about 1 nm to about 100 nm, or about 1 nm to about80 nm, or about 1 nm to about 70 nm, or about 1 nm to about 50 nm, or 50nm to about 400 nm, or about 50 nm to about 350 nm, or about 50 nm toabout 300 nm, or about 50 nm to about 250 nm, or about 50 nm to about200 nm, or about 50 nm to about 150 nm, or about 50 nm to about 100 nm,or about 10 nm to about 70 nm, or about 10 nm to about 80 nm, or about10 nm to about 100 nm, or about 100 nm to about 500 nm, or about 100 nmto about 600 nm, or about 100 nm to about 700 nm, or about 100 nm toabout 800 nm, or about 100 nm to about 900 nm, or about 100 nm to about1000 nm, or about 400 nm to about 500 nm, or about 400 nm to about 600nm, or about 400 nm to about 700 nm, or about 400 nm to about 800 nm, orabout 400 nm to about 900 nm, or about 400 nm to about 1000 nm. Incertain embodiments, the transition metal dichalcogenide nanostructureshave an average size between from about 1 nm to about 200 nm. In certainother embodiments, the transition metal dichalcogenide nanostructureshave an average size between from about 1 nm to about 400 nm. In certainother embodiments, the transition metal dichalcogenide nanostructureshave an average size between from about 400 nm to about 1000 nm. Incertain embodiments, the transition metal dichalcogenide nanostructuresare nanoflakes having an average size between from about 1 nm to about200 nm. In certain other embodiments, the transition metaldichalcogenide nanoflakes have an average size between from about 1 nmto about 400 nm. In certain other embodiments, the transition metaldichalcogenide nanoflakes have an average size between from about 400 nmto about 1000 nm.

In certain embodiments of the methods and devices as otherwise describedherein, transition metal dichalcogenide nanoflakes have an averagethickness between about 1 nm and about 100 μm (e.g., about 1 nm to about10 μm, or about 1 nm to about 1 μm, or about 1 nm to about 1000 nm, orabout 1 nm to about 400 nm, or about 1 nm to about 350 nm, or about 1 nmto about 300 nm, or about 1 nm to about 250 nm, or about 1 nm to about200 nm, or about 1 nm to about 150 nm, or about 1 nm to about 100 nm, orabout 1 nm to about 80 nm, or about 1 nm to about 70 nm, or about 1 nmto about 50 nm, or about 50 nm to about 400 nm, or about 50 nm to about350 nm, or about 50 nm to about 300 nm, or about 50 nm to about 250 nm,or about 50 nm to about 200 nm, or about 50 nm to about 150 nm, or about50 nm to about 100 nm, or about 10 nm to about 70 nm, or about 10 nm toabout 80 nm, or about 10 nm to about 100 nm, or about 100 nm to about500 nm, or about 100 nm to about 600 nm, or about 100 nm to about 700nm, or about 100 nm to about 800 nm, or about 100 nm to about 900 nm, orabout 100 nm to about 1000 nm, or about 400 nm to about 500 nm, or about400 nm to about 600 nm, or about 400 nm to about 700 nm, or about 400 nmto about 800 nm, or about 400 nm to about 900 nm, or about 400 nm toabout 1000 nm); and average dimensions along the major surface of about20 nm to about 100 μm (e.g., about 20 nm to about 50 μm, or about 20 nmto about 10 μm, or about 20 nm to about 1 μm, or about 50 nm to about100 μm, or about 50 nm to about 50 μm, or about 50 nm to about 10 μm, orabout 50 nm to about 1 μm, or about 100 nm to about 100 μm, or about 100nm to about 50 μm, or about 100 nm to about 10 μm, or about 100 nm toabout 1 μm), The aspect ratio (largest major dimension:thickness) of thenanoflakes can be on average, for example, at least about 5:1, at leastabout 10:1 or at least about 20:1. For example, in certain embodimentsthe transition metal dichalcogenide nanoflakes have an average thicknessin the range of about 1 nm to about 1000 nm (e.g., about 1 nm to about100 nm), average dimensions along the major surface of about 50 nm toabout 10 μm, and an aspect ratio of at least about 5:1.

In some embodiments of the methods and devices as otherwise describedherein, the first electrolyte comprises at least one helper catalyst.The person of ordinary skill in the art will appreciate that the term“helper catalyst” refers to an organic molecule or mixture of organicmolecules that does at least one of the following: (a) speeds up thecarbon dioxide reduction reaction, or (b) lowers the overpotential ofthe carbon dioxide reduction reaction, without being substantiallyconsumed in the process. The helper catalysts useful in the methods andthe compositions of the disclosure are described in detail inInternational Application Nos. PCT/US2011/030098 (published as WO2011/120021) and PCT/US2011/042809 (published as WO 2012/006240) and inU.S. Publication No. 2013/0157174, each of which is hereby incorporatedherein by reference in its entirety. In certain embodiments, the helpercatalyst is a compound comprising at least one positively chargednitrogen, sulfur, or phosphorus group (for example, a phosphonium or aquaternary amine). Aqueous solutions including one or more of: ionicliquids, deep eutectic solvents, amines, and phosphines; includingspecifically imidazoliums (also called imidazoniums), pyridiniums,pyrrolidiniums, phosphoniums, ammoniums, choline sulfoniums, prolinates,and methioninates can form complexes with (CO₂)⁻, and as a result, canserve as the helper catalysts. Specific examples of helper catalystsinclude, but are not limited to, one or more of acetylcholines,alanines, aminoacetonitriles, methylammoniums, arginines, asparticacids, threonines, chloroformamidiniums, thiouroniums, quinoliniums,pyrrolidinols, serinols, benzamidines, sulfamates, acetates, carbamates,inflates, and cyanides. These examples are meant for illustrativepurposes only, and are not meant to limit the scope of the presentdisclosure. Aqueous solutions including the helper catalysts describedherein can be used as the electrolyte. Such aqueous solutions caninclude other species, such as acids, bases and salts, to provide thedesired electrochemical and physicochemical properties to theelectrolyte as would be evident to the person of ordinary skill in theart.

In certain embodiments, the helper catalysts of the disclosure include,but are not limited to imidazoliums, pyridiniums, pyrrolidiniums,phosphoniums, ammoniums, sulfoniums, prolinates, and methioninatessalts. The anions suitable to form salts with the cations of the helpercatalysts include, but are not limited to C₁-C₆ alkylsulfate, tosylate,methanesulfonate, bis(trifluoromethylsulfonyl)imide,hexafluorophosphate, tetrafluoroborate, triflate, halide, carbamate, andsulfamate. In particular embodiments, the helper catalysts may be a saltof the cations selected from those in Table 1.

TABLE 1 Cationic Helper Catalysts

wherein R₁-R₁₂ are independently selected from the group consisting ofhydrogen, —OH, linear aliphatic C₁-C₆ group, branched aliphatic C₁-C₆group, cyclic aliphatic C₁-C₆ group, —CH₂OH, —CH₂CH₂OH, —CH₂CH₂CH₂OH,—CH₂CHOHCH₃, —CH₂COH, —CH₂CH₂COH, and —CH₂COCH₃.

In certain embodiments, the helper catalyst of the methods andcompositions of the disclosure is imidazolium salt of formula:

wherein R₁, R₂, and R₃ are independently selected from the groupconsisting of hydrogen, linear aliphatic C₁-C₆ group, branched aliphaticC₁-C₆ group, and cyclic aliphatic C₁-C₆ group. In other embodiments, R₂is hydrogen, and R₁ and R₃ are independently selected from linear orbranched C₁-C₄ alkyl. In particular embodiments, the helper catalyst ofthe disclosure is 1-ethyl-3-methylimidazolium salt. In otherembodiments, the helper catalyst of the disclosure is1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF₄).

In some embodiments, the helper catalyst may be neutral organics, suchas 2-amino alcohol derivatives, isoetarine derivatives, andnorepinepherine derivatives. These examples are meant for illustrativepurposes only, and are not meant to limit the scope of the presentdisclosure.

Of course, not every substance that forms a complex with (CO₂)⁻ will actas a helper catalyst. When an intermediate binds to a catalyst, thereactivity of the intermediate decreases. If the intermediate bonds toostrongly to the catalyst, the intermediate will become unreactive, sothe substance will not be effective. The person of ordinary skill in theart will understand that this can provides a key limitation onsubstances that act as helper catalysts, and will select the helpercatalyst accordingly.

In general, a person of skill in the art can determine whether a givenionic liquid is a co-catalyst for a reaction (R) catalyzed by TM DC asfollows:

-   -   (a) fill a standard 3 electrode electrochemical cell with the        electrolyte commonly used for reaction R. Common electrolytes        include such as 0.1 M sulfuric acid or 0.1 M KOH in water can        also be used;    -   (b) mount the TMDC into the 3 electrode electrochemical cell and        an appropriate counter electrode;    -   (c) run several CV cycles to clean the cell;    -   (d) measure the reversible hydrogen electrode (RHE) potential in        the electrolyte;    -   (e) load the reactants for the reaction R into the cell, and        measure a CV of the reaction R, noting the potential of the peak        associated with the reaction R;    -   (f) calculate VI, which is the difference between the onset        potential of the peak associated with reaction and RHE;    -   (g) calculate VIA, which is the difference between the maximum        potential of the peak associated with reaction and RHE;    -   (h) add 0.0001 to 99.9999 weight % of the ionic liquid to the        electrolyte;    -   (i) measure RHE in the reaction with ionic liquid;    -   (j) measure the CV of reaction R again, noting the potential of        the peak associated with the reaction R;    -   (k) calculate V2, which is the difference between the onset        potential of the peak associated with reaction and RHE; and    -   (l) calculate V2A, which is the difference between the maximum        potential of the peak associated with reaction and RHE.

If V2<V1 or V2A<VIA at any concentration of the ionic liquid (e.g.,between 0.0001 and 99.9999 weight %), the ionic liquid is a co-catalystfor the reaction.

The person of skill in the art will also recognize that the benefits ofthe helper catalyst may be realized at small amount of the helpercatalyst relative to the transition metal dichalcogenide. One can obtainan estimate of the helper catalyst amount needed to change the reactionfrom a Pease study (“The Catalytic Combination of Ethylene and Hydrogenin the Presence of Metallic Copper III. Carbon Monoxide as a CatalystPoison” J. Am. Chem. Soc., 1925, 47(5), pp 1235-1240), which isincorporated into this disclosure by reference in its entirety) of theeffect of carbon monoxide (CO) on the rate of ethylene hydrogenation oncopper. Pease found that 0.05 cc (62 micrograms) of carbon monoxide (CO)was sufficient to almost completely poison a 100 gram catalyst towardsethylene hydrogenation. This corresponds to a poison concentration of0.0000062% by weight of CO in the catalyst. Those familiar with thetechnology involved here know that if 0.0000062% by weight of the poisonin a catalytically active element-poison mixture could effectivelysuppress a reaction, then as little as 0.0000062% by weight of thehelper catalyst relative to the amount of the transition metaldichalcogenide could enhance a reaction. This provides an example of alower limit to the helper catalyst concentration relative to thetransition metal dichalcogenide. Thus, in certain embodiments, thehelper catalyst is present from about 0.000005 weight % to about 50weight % relative to the weight of transition metal dichalcogenide. Insome other embodiments, the amount of the helper catalyst is betweenabout 0.000005 weight % to about 20 weight %, or about 0.000005 weight %to about 10 weight %, or about 0.000005 weight % to about 1 weight %, orabout 0.000005 weight % to about 0.5 weight %, or about 0.000005 weight% to about 0.05 weight %, or about 0.00001 weight % to about 20 weight%, or about 0.00001 weight % to about 10 weight %, or about 0.00001weight % to about 1 weight %, or about 0.00001 weight % to about 0.5weight %, or about 0.00001 weight % to about 0.05 weight %, or about0.0001 weight % to about 20 weight %, or about 0.0001 weight % to about10 weight %, or about 0.0001 weight % to about 1 weight %, or about0.0001 weight % to about 0.5 weight %, or about 0.0001 weight % to about0.05 weight %.

Further, the helper catalyst may be dissolved in water or other aqueoussolution, a solvent for the reaction, an electrolyte, an acidicelectrolyte, a buffer solution, an ionic liquid, an additive to acomponent of the system, or a solution that is bound to at least one ofthe catalysts in a system. These examples are meant for illustrativepurposes only, and are not meant to limit the scope of the presentdisclosure.

In some embodiments of the methods and devices as otherwise describedherein, the first electrolyte is an aqueous solution. In certainembodiments, the first electrolyte is an aqueous solution comprising theat least one helper catalyst. In some embodiments, the helper catalystis present in the aqueous solution in a concentration within the rangeof about 5 vol. % to about 75 vol. %, e.g., about 10 vol. % to about 75vol. %, or about 15 vol. % to about 75 vol. %, or about 20 vol. % toabout 75 vol. %, or about 25 vol. % to about 75 vol. %, or about 30 vol.% to about 75 vol. %, or about 35 vol. % to about 75 vol. %, or about 40vol. % to about 75 vol. %, or about 45 vol. % to about 75 vol. %, orabout 30 vol. % to about 70 vol. %, or about 35 vol. % to about 65 vol.%, or about 40 vol. % to about 60 vol. %, or about 45 vol. % to about 55vol. %, or about 5 vol. %, or about 10 vol. %, or about 15 vol. %, orabout 20 vol. %, or about 25 vol. %, or about 30 vol. %, or about 35vol. %, or about 40 vol. %, or about 45 vol. %, or about 50 vol. %, orabout 55 vol. %, or about 60 vol. %, or about 65 vol. %, or about 70vol. %, or about 75 vol. %.

The person of ordinary skill in the art will appreciate that the firstelectrolyte of the methods and devices as otherwise described herein mayfurther include, e.g., nonaqueous solvents, a buffer solution, anadditive to a component of the system, or a solution that is bound to acatalyst included in the first compartment. In certain embodiments ofthe methods and devices as otherwise described herein, the firstelectrolyte may further comprise other species, such as acids, bases,and salts. The inclusion of such other species would be evident to theperson of ordinary skill in the art depending on the desiredelectrochemical and physicochemical properties of the first electrolyte,and are not meant to limit the scope of the present disclosure.

The devices and methods of the disclosure involve reducing CO₂. Theperson of ordinary skill in the art will appreciate that, e.g., inwater, CO₂ may form chemical derivatives such as carbonic acid,bicarbonate, or carbonate. As used herein, CO₂ and such derivatives maybe referred to interchangeably, i.e., reference to CO₂ reduction in anaqueous solution may also refer to carbonate, bicarbonate, or carbonicacid reduction in an aqueous solution. In some embodiments of themethods and devices as otherwise described herein, a reactant comprisingCO₂, carbonate, or bicarbonate is fed into the first compartment of theelectrochemical device. For example, gaseous CO₂ may be continuouslybubbled through the first compartment. A voltage is applied to the firstcompartment, i.e., upon exposure of the photovoltaic cell to lightirradiation, and the CO₂ reacts to form new chemical compounds. As theperson of ordinary skill in the art will recognize, CO₂ (as well ascarbonate or bicarbonate) may be reduced into various useful chemicalproducts, including but not limited to CO, syngas (mixture of CO andH₂), OH⁻, HCO⁻, H₂CO, (HCO₂)⁻, H₂CO₂, CH₃OH, CH₄, C₂H₄, CH₃CH₂OH,CH₃CO⁻, CH₃COOH, C₂H₆, O₂, H₂, (COOH)₂, and (COO⁻)₂. In certainembodiments, CO₂ may be reduced to form CO, H₂, or a mixture of CO andH₂.

Advantageously, the carbon dioxide used in the embodiments of thedisclosure can be obtained from any source, e.g., an exhaust stream fromfossil-fuel burning power or industrial plants, from geothermal ornatural gas wells or the atmosphere itself. In certain embodiments,carbon dioxide is anaerobic. In other embodiments, carbon dioxide isobtained from concentrated point sources of its generation prior to itsrelease into the atmosphere. For example, high concentration carbondioxide sources are those frequently accompanying natural gas in amountsof 5 to 50%, those from flue gases of fossil fuel (coal, natural gas,oil, etc.) burning power plants, and nearly pure CO₂ exhaust of cementfactories and from fermenters used for industrial fermentation ofethanol. Certain geothermal steams also contain significant amounts ofCO₂. In other words, CO₂ emissions from varied industries, includinggeothermal wells, can be captured on-site. Separation of CO₂ from suchexhausts is well-known. Thus, the capture and use of existingatmospheric CO₂ in accordance with embodiments of the disclosure allowsCO₂ to be a renewable and unlimited source of carbon.

In some embodiments of the methods and devices as otherwise describedherein, the reduction of carbon dioxide may be initiated at high currentdensities. For example, in certain embodiments, the current density ofcarbon dioxide reduction is at least 30 mA/cm², or at least 40 mA/cm²,or at least 50 mA/cm², or at least 55 mA/cm², or at least 60 mA/cm², orat least 65 mA/cm². In one embodiment, the current density of carbondioxide reduction is between about 30 mA/cm² and about 130 mA/cm², orabout 30 mA/cm² and about 100 mA/cm², or about 30 mA/cm² and about 80mA/cm², or about 40 mA/cm² and about 130 mA/cm², or about 40 mA/cm² andabout 100 mA/cm²,or about 40 mA/cm² and about 80 mA/cm², or about 50mA/cm² and about 70 mA/cm², or about 60 mA/cm² and about 70 mA/cm², orabout 63 mA/cm² and about 67 mA/cm², or about 60 mA/cm², or about 65mA/cm², or about 70 mA/cm².

In some embodiments of the methods and devices as otherwise describedherein, the reduction of carbon dioxide may be initiated at lowoverpotential. For example, in certain embodiments, the initiationoverpotential is less than about 200 mV. In other embodiments, theinitiation overpotential is less than about 100 mV, or less than about90 mV, or less than about 80 mV, or less than about 75 mV, or less thanabout 70 mV, or less than about 65 mV, or less than about 60 mV, or lessthan about 57 mV, or less than about 55 mV, or the initiationoverpotential is within the range of about 50 mV to about 100 mV, orabout 50 mV to about 90 mV, or about 50 mV to about 80 mV, or about 50mV to about 75 mV, or about 50 mV to about 70 mV, or about 50 mV toabout 65 mV, or about 50 mV to about 60 mV. In some embodiments, thereduction of carbon dioxide is initiated at overpotential of about 50 mVto about 57 mV, or about 51 mV to about 57 mV, or about 52 mV to about57 mV, or about 52 mV to about 55 mV, or about 53 mV to about 55 mV, orabout 53 mV, or about 54 mV, or about 55 mV.

The methods described herein can be performed at a variety of pressuresand temperatures, and a person of skill in the art would be able tooptimize these conditions to achieve the desired performance. Forexample, in certain embodiments, the methods of the disclosure areperformed at a pressure in the range of about 0.1 atm to about 2 atm, orabout 0.2 atm to about 2 atm, or about 0.5 atm to about 2 atm, or about0.5 atm to about 1.5 atm, or about 0.8 atm to about 2 atm, or about 0.9atm to about 2 atm, about 0.1 atm to about 1 atm, or about 0.2 atm toabout 1 atm, or about 0.3 atm to about 1 atm, or about 0.4 atm to about1 atm, or about 0.5 atm to about 1 atm, or about 0.6 atm to about 1 atm,or about 0.7 atm to about 1 atm, or about 0.8 atm to about 1 atm, orabout 1 atm to about 1.5 atm, or about 1 atm to about 2 atm. In oneparticular embodiment, the methods of the disclosure are carried at apressure of about 1 atm. In other embodiments, the methods of thedisclosure are carried out at a temperature within the range of about 0°C. to about 50° C., or of about 10° C. to about 50° C., or of about 10°C. to about 40° C., or of about 15° C. to about 35° C., or of about 20°C. to about 30° C., or of about 20° C. to about 25° C., or at about 20°C., or at about 21° C., or at about 22° C., or at about 23° C., or atabout 24° C., or at about 25° C. In one particular embodiment, themethods of the disclosure are carried out at a temperature of about 20°C. to about 25° C. The methods of the disclosure may last, for example,for a time within the range of about several minutes to several days andmonths.

Advantageously, in certain embodiments the methods described herein canbe operated at a Faradaic efficiency (FE) within the range of 0 to 100%for the reduction of carbon dioxide to CO. In some embodiments, theFaradaic efficiency of the carbon dioxide-to-CO reduction is at leastabout 3%, or at least about 5%, or at least about 8%, or at least about10%, or at least about 20%, or at least about 25%, or at least about50%, or at least about 60%, or at least about 70%, or at least about75%, or at least about 80%, or at least about 85%.

The person of ordinary skill in the art will appreciate that the cathodeof the first compartment may comprise any of a number of conductivematerials known in the art. In some embodiments, the cathode comprises,e.g., copper, aluminum, carbon black, or stainless steel. In someembodiments, the cathode comprises stainless steel. The person ofordinary skill in the art will further appreciate that the at least onetransition metal dichalcogenide may be contacted with the cathode by avariety of means. For example, in some embodiments, the transition metaldichalcogenide may be disposed on the cathode. In some embodiments, thetransition metal dichalcogenide is disposed on a porous member. Theporous member may be electrically conductive, in which case the porousmember may be in electrical contact with the cathode. In cases where theporous member is not electrically conductive, the person of ordinaryskill in the art can arrange for the electrical connection of thecathode to be made to some other part of the at least one transitionmetal dichalcogenide.

In some embodiments of the methods as otherwise described herein, the atleast one transition metal dichalcogenide is coated onto the cathode ata thickness of, e.g., up to 1000 μm. The person of ordinary skill in theart will appreciate that the thickness of the at least one transitionmetal dichalcogenide may be any convenient thickness, provided CO₂ canbe reduced in the electrochemical device.

In the methods and devices of the disclosure, the second compartmentincludes an anode in contact with at least one water oxidizing catalyst.As used herein, the term “water oxidizing catalyst” refers to a compoundcapable of catalyzing the reaction:

2H₂O→O₂+4H⁺,

and may be used interchangeably with “oxygen evolving catalyst.” In someembodiments of the methods and devices as otherwise described herein,the water oxidizing catalyst comprises cobalt, e.g., Co³⁺.

In the methods and devices of the disclosure, the second compartmentincludes water and a second electrolyte. In some embodiments of themethods and devices as otherwise described herein, the secondelectrolyte and the water comprise an aqueous solution. In someembodiments, the aqueous solution comprises phosphate. In someembodiments, the phosphate comprises potassium phosphate, e.g., KH₂PO₄.In some embodiments, the phosphate is present in the aqueous solution ina concentration within the range of about 0.01 mM to about 100 mM, e.g.,about 0.05 mM to about 50 mM, or about 0.05 mM to about 10 mM, or about0.05 mM to about 5 mM, or about 0.05 mM to about 1 mM, or about 0.05 mMto about 0.9 mM, or about 0.05 mM to about 0.8 mM, or about 0.05 mM toabout 0.7 mM, or about 0.05 mM to about 0.6 mM, or about 0.05 mM toabout 0.5 mM, or about 0.05 mM to about 0.45 mM, or about 0.1 mM toabout 0.4 mM, or about 0.15 mM to about 0.35 mM, or about 0.2 mM toabout 0.3 mM.

The person of ordinary skill in the art will appreciate that the secondelectrolyte of the methods and devices as otherwise described herein mayfurther include, e.g., nonaqueous solvents, a buffer solution, anadditive to a component of the system, or a solution that is bound to acatalyst included in the second compartment. In certain embodiments ofthe methods and devices as otherwise described herein, the secondelectrolyte may further comprise other species, such as acids, bases,and salts. The inclusion of such other species would be evident to theperson of ordinary skill in the art depending on the desiredelectrochemical and physicochemical properties of the secondelectrolyte, and are not meant to limit the scope of the presentdisclosure.

The person of ordinary skill in the art will appreciate that the anodeof the second compartment may comprise any of a number of conductivematerials known in the art. In some embodiments, the anode comprises,e.g., copper, aluminum, carbon black, or stainless steel. In someembodiments, the anode comprises indium tin oxide (ITO). The person ofordinary skill in the art will further appreciate that the at least onewater oxidation catalyst may be contacted with the anode by a variety ofmeans. For example, in some embodiments, the water oxidation catalystmay be disposed on the cathode. In some embodiments, the water oxidationcatalyst is disposed on a porous member. The porous member may beelectrically conductive, in which case the porous member may be inelectrical contact with the anode. In cases where the porous member isnot electrically conductive, the person of ordinary skill in the art canarrange for the electrical connection of the anode to be made to someother part of the at least one water oxidation catalyst.

In some embodiments of the methods as otherwise described herein, the atleast one water oxidation catalyst is coated onto the cathode at athickness of, e.g., up to 1000 μm. The person of ordinary skill in theart will appreciate that the thickness of the at least one wateroxidation catalyst may be any convenient thickness, provided water canbe oxidized in the electrochemical device.

In the methods and devices of the disclosure, the electrochemical deviceincludes at least one photovoltaic cell. The person of ordinary skill inthe art will appreciate that the photovoltaic cell may provide theelectrical energy for the electrochemical reduction of carbon dioxideand the oxidation of water. The person of ordinary skill in the art willappreciate that the photovoltaic cell may be any of a variety of typesand/or arrangements of photovoltaic cells, provided the potentialsupplied to the cathode and anode is sufficient to drive carbon dioxidereduction and water oxidation in the electrochemical device.

In some embodiments of the methods and devices as otherwise describedherein, the at least one photovoltaic cell is a multi-junctionphotovoltaic cell. In some embodiments, the electrochemical deviceincludes two or more photovoltaic cells connected in series. Forexample, in certain embodiments, the electrochemical device comprisestwo multi-junction photovoltaic cells connected in series (See, e.g.,FIG. 25).

In some embodiments of the methods and devices as otherwise describedherein, the at least one photovoltaic cell comprises Si or Ge. In someembodiments, the at least one photovoltaic cell comprises a layercomprising amorphous Si. In some embodiments, the at least onephotovoltaic cell comprises a layer comprising amorphous SiGe. In someembodiments, the at least one photovoltaic cell is a multi-junction cellphotovoltaic cell comprising one or more layers comprising amorphous Si,and one or more layers comprising amorphous SiGe. For example, incertain embodiments, the electrochemical device includes two identicalmulti-junction photovoltaic cells connected in series, each cellcomprising one layer comprising amorphous Si and two layers comprisingamorphous SiGe.

In some embodiments of the methods and devices as otherwise describedherein, the at least one photovoltaic cell is capable of providing atleast about 2.5 V across the cell, e.g., at least about 2.6 V, or atleast about 2.7 V, or at least about 2.8 V, or at least about 2.9 V, orat least about 3 V. In some embodiments, the at least one photovoltaiccell is capable of operating with an efficiency of at least about 4%,e.g. at least about 4.5%, or at least about 5%, or at least about 5.5%,or at least about 6%.

In the methods and devices of the disclosure, the first compartment andthe second compartment are in ionic contact. As used herein, the term“ionic contact” refers to the ability to transport ions from one area toa second area. For example, ions may be transported from a firstcompartment to a second compartment in ionic contact therewith. Ioniccontact may be selective for ions, e.g., by limiting transport ofneutral molecules. Ionic contact may be selective for a specific charge,e.g., selective for positive ions, or for a specific ion, e.g.,selective for protons. The person of ordinary skill in the art willappreciate that there exists in the art a variety of means for providingionic contact, e.g., between two compartments, such as, for example,ion-conductive membranes, proton-conductive membranes, etc.

In some embodiments of the methods and devices of the disclosure, thefirst and second compartments are in general contained and physicallyseparated, e.g., by glass, steel, indium tin oxide, etc., but in partare separated by an ion-conductive (i.e., ion-exchange) membrane, e.g.,a proton-conductive membrane. In some embodiments, the first and secondcompartments are physically contained, e.g., by glass, steel, indium tinoxide, etc., but are separated only by an ion-conductive membrane, e.g.,a proton-conductive membrane. The person of ordinary skill in the artwill appreciate that the area of ionic contact (e.g., the area of aproton-conductive membrane) may be optimized to achieve a desired effectin the electrochemical device.

In some embodiments of the methods and devices as otherwise describedherein, the first and second compartments are in ionic contact through aproton-conductive membrane. In some embodiments, the proton-conductivemembrane is a polymer electrolyte (i.e., an ionomer) membranecomprising, e.g., a sulfonated fluoropolymer. In some embodiments, thefirst and second compartments are in ionic contact through atetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acidcopolymer (i.e., Nafion).

The person of ordinary skill in the art will appreciate that thecompartments and at least one photovoltaic cell of the electrochemicaldevice may be arranged in a variety of different ways such that the atleast one photovoltaic cell is in electrical contact with the anode andcathode and the first and second compartments are in ionic contact. Insome embodiments, the first and second compartments are in generalphysically separated by the photovoltaic cell and, in part separated byan ion-conductive membrane, e.g., a proton-conductive membrane, whereinthe anode and cathode are disposed on and in electrical contact with thephotovoltaic cell. One such embodiment is shown in schematic view inFIG. 1. Electrochemical device 100 comprises a first compartment 120including at least one transition metal dichalcogenide 122 disposed on acathode 121, which cathode is disposed on at least one photovoltaic cell130. Device 100 also comprises a second compartment 140 including atleast one water oxidizing catalyst 142 disposed on an anode 141, whichanode is disposed on cell 130. Compartments 120 and 140 include a firstelectrolyte 123 and a second electrolyte 143, respectively, and are inionic contact through an ion-conductive membrane 150. The person ofordinary skill in the art will appreciate that, in such a configuration,the substrate of the anode or cathode may also function as the substrateof the photovoltaic cell.

In some embodiments, the first and second compartments may be separatedonly by an ion-conductive membrane, e.g., a proton-conductive membrane,wherein the cathode and anode of the first and second compartments,respectively, are electrically connected to the at least onephotovoltaic cell by conductive wires. One such embodiment is shown inschematic view in FIG. 2. Electrochemical device 200 comprises a firstcompartment 220 including at least one transition metal dichalcogenide222 disposed on a cathode 221. Device 200 also comprises a secondcompartment 240 including at least one water oxidizing catalyst 242disposed on an anode 241. Compartments 220 and 240 include a firstelectrolyte 223 and a second electrolyte 243, respectively, and areseparated by, and in ionic contact through, an ion-conductive membrane250. Anode 241 and cathode 221 are in electrical contact withphotovoltaic cell 230 through wires 261 and 262.

Another aspect of the disclosure is an electrochemical device having afirst and second compartment at least one photovoltaic cell, wherein thefirst compartment includes a cathode in electrical contact with the atleast one transition metal dichalcogenide, a first electrolyte, andcarbon dioxide, carbonic acid, or a carbonic acid salt; the secondcompartment includes an anode in electrical contact with at least onewater oxidizing catalyst, a second electrolyte, and water; and whereinthe at least one photovoltaic cell is in electrical contact with theanode and the cathode, and the first compartment is in ionic contactwith the second compartment.

In some embodiments of the electrochemical device, the firstcompartment, cathode, transition metal dichalcogenide, firstelectrolyte, second compartment, anode, water oxidizing catalyst, secondelectrolyte, and photovoltaic cell are as otherwise described herein. Insome embodiments of the electrochemical device as otherwise describedherein, the first electrolyte comprises at least one helper catalyst. Insome embodiments, the at least one transition metal dichalcogenide isMoS₂ or WSe₂. In some embodiments, the at least one transition metaldichalcogenide is in nanoflake, nanosheet, or nanoribbon form. In someembodiments, the helper catalyst is 1-ethyl-3-methylimidazoliumtetrafluoroborate. In some embodiments, the first electrolyte is anaqueous solution. In some embodiments, the helper catalyst is present inthe aqueous solution in a concentration within the range of about 25vol. % to about 75 vol. %. In some embodiments, the electrochemicaldevice as otherwise described herein is for use in reducing carbondioxide and oxidizing water.

Another aspect of the disclosure is a method of electrochemicallyreducing carbon dioxide in an electrochemical cell, comprisingcontacting the carbon dioxide with at least one transition metaldichalcogenide in the electrochemical cell and at least one helpercatalyst and applying a potential to the electrochemical cell, whereinthe at least one transition metal dichalcogenide is WSe₂ or WS₂. In someembodiments of the method, the helper catalyst is as otherwise describedherein. In some embodiments, the electrochemical cell comprises acathode as otherwise described herein, wherein the cathode is in contactwith the at least one transition metal dichalcogenide. In someembodiments, the electrochemical cell comprises a first electrolyte asotherwise described herein, wherein the first electrolyte comprises theat least one helper catalyst.

Another aspect of the disclosure is a method of electrochemicallyreducing carbon dioxide comprising providing an electrochemical cellhaving a cathode in contact with at least one transition metaldichalcogenide, and a first electrolyte comprising at least one helpercatalyst in contact with the cathode and the at least one transitionmetal dichalcogenide, wherein the at least one transition metaldichalcogenide is WSe₂ or WS₂; providing carbon dioxide to theelectrochemical cell; and applying a potential to the electrochemicalcell. In some embodiments, the cathode, first electrolyte, and helpercatalyst are as otherwise described herein. In some embodiments, thetransition metal dichalcogenide is in bulk form. In some embodiments,the transition metal dichalcogenide is in nanoparticle form, asotherwise described herein. In some embodiments, the transition metaldichalcogenide nanoparticles have an average size between about 1 nm and400 nm. In some embodiments, the transition metal dichalcogenide is innanoflake, nanosheet, or nanoribbon form, as otherwise described herein.In some embodiments, the transition metal dichalcogenide nanoflakes,nanosheets, or nanoribbons have an average size between about 1 nm and400 nm. In some embodiments, the helper catalyst is as otherwisedescribed herein. In some embodiments, the helper catalyst is1-ethyl-3-methylimidazolium tetrafluoroborate.

Another aspect of the disclosure is an electrochemical cell having acathode in contact with at least one transition metal dichalcogenide anda first electrolyte comprising at least one helper catalyst, wherein theat least one transition metal dichalcogenide is WSe₂ or WS₂. In someembodiments of the cell, the cathode, first electrolyte, and cathode areas otherwise described herein. In some embodiments, the firstelectrolyte is an aqueous solution of the helper catalyst. In someembodiments, the helper catalyst is 1-ethyl-3-methylimidazoliumtetrafluoroborate. In some embodiments the cell as otherwise describedherein, the cell is for use in reducing carbon dioxide.

EXAMPLES

The Examples that follow are illustrative of specific embodiments of thedisclosure, and various uses thereof. They are set forth for explanatorypurposes only, and are not to be taken as limiting the disclosure.

Example 1 Transition Metal Dichalcogenide Preparation

Transition metal dichalcogenides (e.g., MoS₂, MoSe₂, WS₂, and WSe₂) weresynthesized through direct reaction of pure forms of the relevantelements followed by a vapor transport process in an evacuated ampule atelevated temperatures. In this method, powders of the transition metalsand chalcogens (>99.99% trace metal basis purity) were mixed in thedesired stoichiometric ratio and loaded into a quartz ampule. The totalloaded weight was about one gram. Each quartz ampule had a 1 cm internaldiameter and a 20 cm length. The ampule was then evacuated with a turbomolecule pump (<10⁻⁶ mbar) and sealed with a hydrogen torch. The ampulewas placed into a two-zone CVD furnace and the temperature of both zoneswas raised to 1080° C. over 1 day. The temperature of the empty end ofthe ampule (the cold zone) was then gradually cooled to 950° C. over 4days, while the other end was maintained at 1080° C., providing singlecrystalline grains with pristine structure via direct vapor transport.The system was slowly cooled to room temperature over 1 day, after whichthe crystalline material (See, e.g., FIG. 3) was removed forcharacterization.

Example 2 Transition Metal Dichalcogenide Nanoflake Preparation

The crystalline grains produced according to Example 1 were ground to apowder. Nanoflakes were formed by sonicating a dispersion of 300 mg ofground transition metal dichalcogenide powder in 60 mL of isopropanol.The dispersion was sonicated for 30 hours, using a sonication probe(Vibra Cell Sonics 130 W). The sonicated dispersions were thencentrifuged for 60 minutes at 2000 rpm, after which the supernatant (thetop two thirds of the centrifuged dispersion) was collected by pipetteand stored in a glass vial. FIG. 4 shows nanoflakes dispersed inisopropanol, after centrifugation.

Example 3 Nanoflake Characterization

Dynamic light scattering (DLS) experiments were performed to measurenanoflake sizes using a NiComp ZLS 380 system configured with a 35 mWsemiconductor laser (670 nm emission) and a thermoelectric temperaturecontrol for samples (held at 25° C.). Nanoflakes dispersed inisopropanol were measured, providing the normal distributions shown inFIG. 5.

The nanoflakes were also characterized by Raman spectroscopy, using aHORIBA LabRAM HR Evolution confocal Raman microscope configured with a532 nm laser source, a 1200 g/mm grating, a Horiba Andor detector, and a100x objective. Laser powers at the sample were held between 1-15 mW.Integration times and averaging parameters were chosen to maximize thesignal-to-noise ratio. Results are shown in FIG. 6.

Finally, WSe₂ nanoflakes were imaged with scanning electron microscopy(SEM) to understand the microscale morphology of the nanoflakes. Sampleswere imaged with a Carl Zeiss SEM instrument integrated in a Rait e-LiNEplus ultra-high resolution electron beam lithography system. Sampleswere kept at a distance of 10 mm from the electron source, held at 10kV. Results are shown in FIG. 7.

Example 4 Three-Electrode Electrochemical Characterization

A two-compartment three-electrode electrochemical cell was used toperform CO₂ reduction reactions (See, FIG. 8). Transition metaldichalcogenide nanoflakes prepared according to Example 2 were drop-castonto a glassy carbon substrate to form the working electrode. Platinumgauze 52 mesh (Alfa Aesar) and Ag/AgCl (BASi) were used as the counterand reference electrodes, respectively. The working electrode, referenceelectrode, and counter electrode (CE) were immersed in an aqueoussolution of 50 vol. % 1-ethyl-3-methylimidazolium tetrafluoroborate(EMIM-BF₄). All potentials are presented with respect to a reversiblehydrogen electrode (RHE), using the following equation:

Potential vs. RHE=Applied Potential vs. Ag/AgCl+0.197 V+(0.0592×pH)

The cathode and anode were separated by an ion-conductive membrane toeliminate potential product oxidation at the anode surface. Allexperiments were performed using a rotating disk electrode (RDE)submerged in the cell. To eliminate any effect of mass transport duringthe reactions, the working electrode was rotated at 1000 rpm. The cellwas connected to a potentiostat (CH Instruments) connected to a computerthrough CH Instruments software. A 6 mm polyethylene tube bubbled CO₂(99.9% UHP, Praxair) through the electrolyte solution for 30 minutesprior to experiments. Results of cyclic voltammetry andchronoamperometry experiments, shown in FIGS. 9-11 and 13, show theperformance of transition metal dichalcogenide nanoflakes in comparisonwith catalysts comprising bulk transition metal dichalcogenides, bulksilver, or silver nanoparticles.

Example 5 pH Characterization of Electrolyte Composition

Table 2, below, shows the pH of the aqueous electrolyte solution of thethree-electrode cell as a function of different concentrations ofEMIM-BF₄.

TABLE 2 pH Value of EMIM-BF₄ Concentrations Water Volume Fraction pH  0%H2O 6.54  5% H2O 4.87 10% H2O 4.54 25% H2O 4.14 50% H2O 3.20 65% H2O3.78 75% H2O 3.98 85% H2O 4.82 90% H2O 5.30 95% H2O 5.98

A working electrode coated with WSe₂ nanoflakes was tested at apotential of −0.764 V vs. RHE in a chronoamperometry experiment carriedout according to Example 4, at various pHs (i.e., differentconcentration of EMIM-BF₄). The results, shown in FIG. 12, indicate thatthe acidity associated with 50 vol % EMIM-BF₄ provided for maximumcatalytic activity.

Example 6 Turn Over Frequency (TOF) Measurement

To further characterize the catalytic activity of the materials ofExamples 1-5, a roughness factor (RF) technique was employed todetermine the number of active edge sites of the materials. Allexperiments were performed using the same surface area, (the catalystloadings for each material were different, however). The RF number ofWSe₂ nanoflakes were estimated by comparing its double layer capacitance(D_(dl)) with a flat standard capacitor of MoS₂ (See, Table 3). Cyclicvoltammetry experiments were performed at different scan rates in 0.5MH₂SO₄ to calculate the C_(dl) of each material (See, FIG. 13). FIG. 14shows the extracted C_(dl) values of 2.6, 2.23, and 3.71 mF/cm² at +0.2V vs. RHE for WSe₂ nanoflakes, bulk MoS₂, and Ag nanoparticles,respectively. As shown in Table 3, RF values of 44, 37, and 148 wereobtained for WSe₂ nanoflakes, bulk MoS₂, and Ag nanoparticles,respectively. The calculated number of active sites for each catalystwas obtained using the following equation:

Density of active sites(sites/cm²)=(Density of active sites of standardsample)×RF

TABLE 3 Active Sites for Example Materials Flat Standard Double LayerCapacitance Capacitance Roughness Active Sites Catalyst (μF/cm²)(mF/cm²) Factor (#) WSe2 NFs 60 2.6 44 5.1 × 1016 Bulk MoS2 60 2.23 374.3 × 1016 Ag NPs 25 3.71 148 4.44 × 1017 

Additionally, the CO formation TOF of active sites for CO₂ reduction byWSe₂ nanoflakes, bulk MoS₂, and Ag nanoparticles in the aqueous EMIM-BF₄electrolyte was calculated at various overpotentials using the followingequation:

CO formation TOF(s ⁻¹)=i ₀(A/cm²)×CO formation FE/{[active sitedensity(sites/cm²)]×[1.602×10⁻¹⁹(C/e ⁻)×[2e⁻/CO₂]}

Results, provided in FIG. 15, show that the CO formation TOF for WSe₂was approximately three orders of magnitude higher than that of Agnanoparticles in the overpotential range of 150 to 650 mV.

Example 7 Gas Chromatography (GC) Analysis

The products of the electrochemical experiments of Example 4 wereanalyzed with gas chromatography (GC) using an SRI 8610C GC systemequipped with a 72 in.×18 in. stainless steel column packed withmolecular sieves. A thermal conductivity detector was used to analyzeand differentiate the injected samples. Ultra-high-purity helium andnitrogen (Praxair) were used as carrier gases for CO and H₂ detection,respectively.

The GC apparatus was calibrated to determine the moles of products(i.e., CO and H₂) using 2, 5, 10 and 20 vol % of CO and H₂ in He and N(99.99% research grade, Praxair), respectively. The known volume ofstandard samples (1 mL) were injected at a constant pressure (10 psi)and temperature (25° C.), using helium and nitrogen as carrier gases forCO and H₂ detection, respectively. A distinct CO peak was apparent at3.83 min., and H₂ was detected at 0.98 min. Calibration curves (See,FIGS. 16-17) were calculated using the integrated peak area and theknown number of moles of CO or H₂ that were injected.

1 mL of the gas products of the electrochemical experiments of Example4, carried out for a desired time (e.g., 10 min.), was injected into theGC instrument using a sample lock syringe. Only CO and H₂ were detected.

In order to identify any other potential carbon-based products, ¹³CO₂was reduced in the electrochemical experiment of Example 4, the productsof which were analyzed using differential electrochemical massspectrometry (DEMS) with a quadrupole detector (HPR-20, purchased fromHiden Analytical Inc.). The DEMS instrument was operated atultra-high-vacuum pressure (1×10⁻⁶ torr) through the analysis. Theproduct stream was injected to the DEMS instrument at a flow rate of0.8-1 mL/min using a quartz coated very-low-flow capillary line.Analysis of the product stream, shown in FIG. 18, indicates that CO wasthe only detectable carbon-based product of the reaction.

Example 8 Faradaic Efficiency (FE) of Transition Metal Dichalcogenides

The Faradaic efficiency (FE) of the transition metal dichalcogenidesmaterials produced according to Example 2 were calculated using thefollowing equation:

FE=100×(moles of product)/[{j(mA/cm²)×t(s)}/nF ]

wherein the number of moles of product is determined according toExample 7, j is the curve area of the plot of current density vs. time,provided in Example 4, n is the number of electrons required for thereduction of CO₂ to CO (i.e., 2), and F is the Faradic number.

The resultant FE values (See, FIGS. 19-22) indicated that CO and H₂ weredominant products of the materials and systems described in thepreceding Examples, at a potential window of 0 to −0.764 V, with anoverall FE of 90±5%. Accordingly, the formation efficiency of otherproducts, e.g., HCOOH, methanol, and other liquid phase products is−10%. These results indicate that at very low potentials, e.g., 0 to−0.2 V, H₂ is the major product, while at higher potentials, e.g., −0.2to −0.764 V, CO becomes the major product. Without being bound by aparticular theory, this difference is believed to be attributable to thedifferences in the CO₂ reduction and hydrogen evolution reaction (HER)mechanisms. In principle, the thermodynamic potential for H₂ evolutionis lower than that for CO₂ reduction. As the applied potential exceedsthe onset potential of CO₂ reduction (−0.164 V), the reaction isactivated, and catalyst sites become occupied by CO₂ reductionintermediates.

The product of the current density and FE of the materials of thepreceding Examples was plotted as a function of overpotential to providean overview of catalytic performance. Results, provided in FIG. 23, showthat the performance of WSe₂ nanoflakes at 100 mV overpotential exceededthat of bulk MoS₂ and Ag nanoparticles under identical conditions by afactor of nearly 60.

Example 9 WSe₂ Nanoflake Stability Investigation

The long-term stability of WSe₂ nanoflakes were investigated in anelectrochemical experiment configured and carried out similarly toExample 4. A magnetic stirrer was placed in the electrolyte solution toeliminate any potential complications due to mass transport. Thestability of the WSe₂-coated electrode was recorded for 27 hours at apotential of −0.364 V (0.254 V overpotential) using a Voltalab PGZ100potentiostat (Radiometer Analytical SAS) calibrated with a RCB200resistor capacitor box. The potentiostat was connected to a PC usingVolta Master (Version 4) software. The results, shown in FIG. 24,indicate that the material is highly stable. The observed spikes are dueprimarily to fluctuations in the flow rate of the CO₂ bubbled throughthe electrolyte solution.

Example 10 Photoelectrochemical Device Configuration

All chemicals were used as received, without any purification, unlessrequired. Cobalt nitrate hexahydrate (Alfa Aesar), potassium basedbuffer solution (0.071 M KPi, pH=7, Sigma-Aldrich), Nafion 117 (10.0cm×10.0 cm, FuelCellsEtc) were used in the following configuration.Triple-junction amorphous-Si solar cells were purchased from Xun-lightCorp. (Toledo, Ohio). The acrylic used for the chambers was purchasedfrom Total Plastics Inc.

A photoelectrochemical chamber was machined from acrylic plastic andassembled with acrylic glue. The transparent chamber was separated intotwo compartments by two tandem amorphous-Si-based triple-junction(a-Si/A-SiGe/A-SiGe) photovoltaic (PV) cell comprising an indium tinoxide (ITO) anode layer disposed on the exposed a-Si layer and astainless steel cathode layer disposed on the exposed a-SiGe layer,connected in series through copper tape and separated by a piece ofnafion membrane (See, FIG. 25).

A cobalt oxygen-evolving catalyst was electrodeposited onto the ITOsurface of the PV cells from a cobalt (II) nitrate hexahydrate solution.The electrodeposition was carried out using a solution prepared bymixing 73 mg of cobalt nitrate hexahydrate in 500 mL of potassiumphosphate (2.6×10⁻⁴ M K⁺, pH=7) using a three-electrode cellconfiguration comprising a platinum mesh counter electrode and a Ag/AgClreference electrode, wherein the ITO layer of the PV cell served as theworking electrode. Electrodeposition was carried out at a potential of1.5 V vs Ag/AgCl for 5 minutes, without stirring and without any i-Rcompensation. The stainless steel layer was covered throughout theelectrodeposition.

WSe₂ nanoflakes were prepared according to Example 2 and suspended inisopropanol. The suspension was drop cast onto the stainless steel anodeof the PV cells, which were allowed to dry completely.

The catalyst-coated anode/PV cell/cathode unit and a section of nafionmembrane (activated by treatment with 5 wt. % KOH) were configured toseparate the two compartments of the device while allowing for ioniccontact between the compartments (See, FIG. 25). The compartment exposedto the cathode and WSe₂ nanoflakes was filled with 100 mL of an aqueoussolution of 50 vol % EMIM-BF₄ (pH=3.23). CO₂ (99.9% UHP, Praxair) wasbubbled through the solution at 1 mL/min for 30 minutes to saturate thesolution. The compartment exposed to the anode and Co catalyst wasfilled with 100 mL of an aqueous potassium phosphate solution (2.6×10⁻⁴M K⁺, pH=7).

Example 11 Photoelectrochemical Device Operation

Upon exposure to light irradiation, the photoelectrochemical deviceconfigured according to Example 10 operates first in a transient regime,after which the device operates in a “steady-state.” Initially, the H⁺concentration in the solution of the anodic compartment is much lowerthan that of the cathodic compartment. Upon exposure to lightirradiation, H⁺ is produced at the anode (i.e., through wateroxidation), lowering the pH of the anodic solution, while the reductionof CO₂ at the cathode consumes available H⁺, increasing the pH of thecathodic solution. During this time, K⁺ ions diffuse through theproton-conductive membrane to compensate for charge imbalance. Afterapproximately 5 minutes, the pH of the solutions of both compartmentsequilibrate at 3.35, at which point the electrochemical device reachessteady-state operation, wherein diffusion of H⁺ from the anodiccompartment to the cathodic compartment overtakes that of K⁺ (See, FIG.26).

K⁺ crossover in the device was quantified using a PerkinElmerInductively Coupled Plasma—Optical Emission Spectroscopy (ICP-OES,Optima 5300DV) instrument. Solution samples were collected at varioustime intervals and diluted by a factor of 5 or 20 using 2% HNO₃, basedon sample volume. Diluted samples were analyzed using an ESI Fastauto-sampler coupled with the ICP-OES device. Measurements demonstratedthat the K⁺ concentration of the aqueous solution of EMIM-BF₄ (i.e., thecathodic solution) reached 1.43×10⁻⁴ M after five minutes of exposure ofthe photovoltaic cell to irradiation, after which the concentrationremained constant. This result is consistent with the slightly increasedpH of the cathodic solution after the same period of operation, whichcorresponds to a change in H⁺ concentration of 1.52×10⁻⁴ M. Theperformance of the device decreases after about 5 hours of continuousoperation.

Example 12 Photoelectrochemical Device Product Analysis

The product stream of the device operated according to Example 11 wasanalyzed according to the method of Example 7. Results, shown in FIG.27, indicated that H₂ and CO were the main products of thephotoelectrochemical reaction. No other detectable carbon based productswere observed during operation.

Example 13 Photoelectrochemical Device Stability Analysis

As evidenced by FIG. 28, the drop in performance of the device operatedaccording to Example 11 after 5 hours of continuous operation isbelieved to be due to the corrosion of the ITO layer disposed on the PVcells. Device performance is restored upon replacement of thecatalyst-coated anode/PV cell/cathode unit. To test the stability of theanodic and cathodic solutions, the catalyst-coated anode/PV cell/cathodeunit was replaced every 4 hours throughout a period of continuousoperation lasting 100 hours. Results, shown in Table 4, indicate thatthe same quantities of CO and H₂ are produced throughout the 100 hourperiod of operation, suggesting that the anodic and cathodic solutionsare highly robust. No significant change to the pH of either solutionwas observed.

TABLE 4 Device Performance Time CO H2 (hours) (mmol) (mmol) 4 3.95880.4468 8 3.9469 0.4069 12 4.0263 0.3969 16 3.9985 0.4348 20 3.95480.3985 24 3.9311 0.4039 28 4.0858 0.3981 32 4.0580 0.3965 36 4.05200.4348 40 4.0719 0.3973 44 3.9628 0.3981 48 4.0144 0.4269 52 3.93490.3881 56 3.9690 0.4098 60 3.9471 0.4315 64 3.9006 0.4237 68 4.03020.4189 72 3.9982 0.4205 76 3.9487 0.4349 80 3.8882 0.4017 84 4.03020.4251 88 4.0501 0.4234 92 4.0144 0.4191 96 3.8859 0.4278 100 3.97140.4338

Example 14 Cathodic Water Production Calculation

The volume of water produced through the device operation of Example 11was calculated. Because water and CO are be produced in a stoichiometricratio of 1:1 at the cathode of the device, the rate of water production,under 1 sun of illumination is known to be 2.75×10⁻⁷ mol/s, or 9.9×10⁻⁴mole of water generated over 1 hour of operation. After 100 hours ofoperation, only 1.8 mL water will have been produced, which will have anegligible effect on the pH or composition of the cathodic solution.Table 5 shows the amount of produced water at different levels ofillumination of the device configured according to Example 10.

TABLE 5 Water Production at the Cathode #sun Water Illumination CO(mol/s) (mol/s) Water (mL/h) 0.5 1.32 × 10⁻⁷ 1.32 × 10⁻⁷ 0.00855 1 2.75× 10⁻⁷ 2.75 × 10⁻⁷ 0.0178 1.5 4.08 × 10⁻⁷ 4.08 × 10⁻⁷ 0.0264 2 5.21 ×10⁻⁷ 5.21 × 10⁻⁷ 0.0338

Example 15 Solar to Fuel Conversion Efficiency (SFE) Calculation

The solar to fuel conversion efficiency (SFE) of the device operation ofExample 11 was calculated using the following equation:

$\eta = \frac{{N_{1}E_{1}} + {N_{2}E_{2}}}{U_{g}A_{cat}}$

wherein N₁ and N₂ are the molar quantities of produced gas per unit time(mol/s) provided by the GC analysis of Example 12, E₁ and E₂ are theenergy densities of the corresponding gas (kJ/mol), which are 283.24 and140 kJ/mol for CO and H₂, respectively, A_(cat) is the overallcatalystic surface area available for the reaction (cm²), which is 18cm², and U_(g) is the total solar irradiance (mW/cm²), which is 100mW/cm² for 1 sun of illumination. FIG. 29 shows the SFE and rate ofproduct formation of the electrochemical device at various levels ofillumination.

To estimate the uncertainty of the calculations, the partial derivativemethod is used to calculate the sensitivity of the SFE values todifferent input parameters. For this purpose, each parameter of theabove equation was perturbed by a small amount (∂x_(i)) around itstypical value (x_(i)) to provide a corresponding change in the extractedSFE (∂η). The dimensionless sensitivities were then calculated using thefollowing equation:

$s_{i} = {\frac{x_{i}}{\eta}\frac{\partial\eta}{\partial x_{i}}}$

The overall uncertainty (u_(η)) was also calculated, using the followingequation:

$\frac{u_{\eta}}{\eta} = \sqrt{\sum\limits_{i}\left( {s_{i} \times \frac{u_{x_{i}}}{x_{i}}} \right)^{2}}$

where u_(x) _(i) is the overall uncertainty of the i^(th) parameteraround its typical value (x_(i)), s_(i) is the sensitivity to thatparticular input, and η is the SFE value.

$\frac{u_{x_{i}}}{x_{i}}$

for N₁ and N₂ was calculated based on the standard deviations of thevalues from three different experiments, which were 0.07 and 0.09,respectively. Because E₁ and E₂ values were literature values,

$\frac{u_{x_{i}}}{x_{i}}$

for these values were considered to be zero. The value of

$\frac{u_{x_{i}}}{x_{i}}$

for U_(g) was based on the fluctuation in the response of the photodiodeduring device operation. The uncertainty in A_(cat) was 0.05 (based onmeasurement with a Vernier caliper). A summary of the uncertaintyanalysis is provided in Table 6. The error bars shown in FIG. 29represent the calculated overall uncertainty values (u_(η)). Theuncertainty values for 0.5, 1, 1.5 and 2 suns of illumination were0.39058, 0.40904, 0.40154, and 0.38885, respectively.

TABLE 6 SFE Uncertainty Analysis c_(i) = |S.| × Input Units x_(i)(values) u_(xi)/x_(i) Sensitivity u_(xi)/x_(i) (ci)²/Σ(ci)² N₁ mol/s2.75 × 10−7 0.07 0.952643 0.0667 0.00445 N₂ mol/s 2.78 × 10−8 0.090.047357 0.0043 1.8 × 10−5 E₁ kJ/ 283.24 0 0.952643 0 0 mol E₂ kJ/ 140 00.047357 0 0 mol U_(g) W/ 100 0.035 0.98039 0.0343 0.0012 cm² A_(cat.)cm² 18 0.05 0.98039 0.049 0.0024

The SFE values over 5 hours and 100 hours of continuous operationaccording to Example 11 were also calculated, and are provided in FIG.30 and Table 7.

TABLE 7 SFE Values of 100-hour Continuous Operation Time (hours) SFE % 44.570 8 4.535 12 4.617 16 4.607 20 4.539 24 4.516 28 4.682 32 4.651 364.665 40 4.667 44 4.548 48 4.620 52 4.512 56 4.561 60 4.549 64 4.494 684.633 72 4.599 76 4.553 80 4.468 84 4.636 88 4.657 92 4.616 96 4.480 1004.577

Example 16 PV Cell Efficiency Measurement

The solar-to-electricity conversion efficiency of a triple-junctionphotovoltaic cell comprising an ITO layer and a cobalt oxygen evolvingcatalyst disposed thereon was measured under one sun of illumination.The voltage was measured directly with a multi-meter while theresistance of the circuit was changed using variable resistors. The opencircuit voltage (V_(OC)), short circuit current, and average fill factorof the cell were 2.12 V, 6.1 mA/cm² and 0.55, respectively. Accordingly,a dry cell efficiency of 7.1% was calculated by dividing the product ofthe three aforementioned parameters by the energy of one sun ofillumination (100 mW/cm²). The V_(OC) of two such cells connected inseries was 3.6 V, which decreased to 3V when submerged in theelectrolyte solutions of Example 10 (the short circuit current remainedconstant). The fill factor was assumed to remain relatively constantunder the configuration and operation parameters of Examples 10 and 11.Accordingly, the efficiency of the PV cell was estimated to be about 6%when submerged in the electrolyte.

Example 17 Electrochemical Impedance Spectroscopy (EIS)

Electrochemical impedance spectroscopy (EIS) experiments were performedusing an electrochemical cell and electrodes configured similarly toExample 4. The Nyquist plot for different CO₂ reduction over-potentials,e.g., 150, 200, 300, 400, and 500 mV, were recorded at a small (10 mV)AC voltage amplitude (to avoid nonlinearity) and over a frequency rangeof 10 to 10⁵ Hz, using a Voltalab PGZ100 potentiostat. An equivalentRandles circuit model was fit to the data to calculate R_(ct) for eachcatalyst system. FIG. 31 shows the recorded Nyquist plots and fittedcurve at an overpotential of 150 mV for WSe₂ nanoflakes, bulk MoS₂, andAg nanoparticles disposed on glassy carbon. FIG. 32 shows the recordedNyquist plots and fitted curve at overpotentials of 150-500 mV for WSe₂nanoflakes disposed on glassy carbon.

Example 18 Ultraviolet Photoelectron Spectroscopy (UPS)

The work function for four transition metal dichalcogenides and Agnanoparticles was measured by ultraviolet photoelectron spectroscopy(UPS) (See, FIG. 32). UPS data were acquired with a Physical ElectronicsPHI 5400 photoelectron spectrometer using He I (21.2 eV) UV radiationand a pass energy of 8.95 eV.

Example 19 Scanning Transmission Electron Microscopy (STEM) images ofWSe₂

Scanning transmission electron microscopy (STEM) images were acquired ona JEAL JEM-ARM200CF instrument operated at 200 kV. Images were acquiredin either high/low angle annular dark field (H/LAADF) or annular brightfield (ABF) mode. FIG. 33 shows STEM images of WSe₂ before and after a27 hour chronoamperometry experiment carried out according to Example 4.

Example 20 X-Ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) experiments were carried out on aThermo Scientific ESCALAB 250Xi instrument equipped with an electronflood and scanning ion gun. Spectra were calibrated to the C1s bindingenergy of 284.8 eV. As shown in FIG. 34, WSe₂ nanoflakes, after a 27hour chronoamperometry experiment carried out according to Example 4,showed a negligible (0.2 eV) change in the W 4f and Se 3d spectrarelative to that of fresh nanoflakes prepared according to Example 2,indicating that the nanoflakes are highly stable, even over prolongedperiods of use.

Example 21 Density Functional Theory (DFT) Calculations

Density functional theory (DFT) calculations were carried out toinvestigate the catalytic properties of transition metal dichalcogenidenanoflakes (e.g., nanoflakes prepared according to Example 2). PeriodicDFT calculations were performed with plane wave basis sets in the VASPpackage. Reaction free energies and density of states (DOS) calculationswere performed on single-layer nanoribbons of the transition metaldichalcogenides truncated with zig-zag edges. FIGS. 35-36 show thecalculated partial density of states (PDOS) of the d band (spin up) ofthe surface bare metal edge atom (Me and W) of MoSe₂, MoS₂, WS₂, andWSe₂ nanoflakes, respectively, in addition to the surface Ag atom ofbulk Ag(111) and Ag₅₅ nanoparticles. FIG. 37 shows the calculated freeenergy diagrams for CO₂ electroreduction to CO on Ag(111), Ag₅₅nanoparticles, MoS₂, WS₂, MoSe₂, and WSe₂ nanoflakes at 0 V RHE.

Both monolayer slabs and nanoribbons of the transition metaldichalcogenides were used to calculate the work functions. For thenanoribbons, each unit well included 4×4 (16 total) metal atoms and 32 Sor Se atoms (for low CO coverage calculations, the unit cell included6×4 metal atoms and 48 S or Se atoms), containing both the metal and theS/Se edges. A 10 A vacuum space was set both on top of the metal edgeand between two nanoribbon periodic images. For the single-layer slabsfor work function calculations, only the minimum atoms to construct aunit cell were used. The Ag(111) surface was constructed by a 4×4×4 slabin a unit cell, with 10 Å vacuum space. A kinetic energy cutoff of 400eV was used for all calculations. All atoms in the system were allowedto relax, while the cell shape and volume were kept fixed. K-point gridsof 3×1×1 and 3×3×1 were used for energy calculations of the nanoribbonsand Ag(111), respectively. K-point grids of 6×1×1 and 6×6×1 were usedfor DOS calculations of the nanoribbons and Ag(111), respectively.r-point was used for gas-phase molecules. For work function calculationsof the monolayer transition metal dichalcogenide slabs, a 10×1×1 K-pointgrid was used. All calculations were spin-polarized calculations.

The effect of the CO coverage on the CO binding energies on the metaledges of the transition metal dichalcogenides was investigated. The DFTresults show that each metal atom on the transition metal dichalcogenidenanoflake edge can bind up to two CO molecules (θ_(co)=2 ML). As shownin Table 8, the binding energies of CO on the metal edge decrease as thecoverage increases. At the highest coverage (θ_(co)=2 ML), the averagebinding energy per second CO on the metal atom becomes smaller than 0.5eV. This suggests that during the catalytic reaction, CO is likely tohave a high coverage (θ_(co)>1 ML) on the metal edges of the transitionmetal dichalcogenides, and second CO molecule on the metal atom caneasily desorb. These results indicate that the catalyst site may have atleast one CO molecule binding the metal atom during most of thecatalytic cycle.

TABLE 8 Calculated Binding Energies CO Coverage 1/6 ML 1 ML 1.25 ML 2 MLMoS₂ 1.27 0.85 0.80 0.27 MoSe₂ 1.20 0.81 0.82 0.31 WS₂ 1.55 1.14 0.880.28 WSe₂ 1.42 1.05 0.90 0.48 For θ_(CO) = 1/6 ML and 1 ML, the valuesare average binding energies per CO molecule; for θ_(CO) = 1.25 ML and 2ML, the values are the average binding energies per second CO on themetal atom.

FIG. 38 shows the calculated work functions for the transitiondichalcogenide monolayers. A clear trend was observed among the workfunctions of the four transition metal dichalcogenides, whereinMoS₂>WS₂>MoSe₂>WSe₂. The calculated work functions of the nanoribbonswere consistently around 0.3 eV lower than those of the monolayers.

We claim:
 1. A method of electrochemically reducing carbon dioxide andoxidizing water in an electrochemical device, the method comprisingproviding an electrochemical device, the device including a first andsecond compartment and at least one photovoltaic cell, wherein the firstcompartment includes a cathode in electrical contact with at least onetransition metal dichalcogenide, a first electrolyte, and carbondioxide, carbonic acid, or a carbonic acid salt; the second compartmentincludes an anode in electrical contact with at least one wateroxidizing catalyst, a second electrolyte, and water; the at least onephotovoltaic cell is in electrical contact with the anode and thecathode; and the first compartment is in ionic contact with the secondcompartment; and exposing the photovoltaic cell to light irradiationsufficient to create a potential difference between the anode and thecathode sufficient to reduce carbon dioxide at the cathode and tooxidize water at the cathode.
 2. A method according to claim 1, whereinthe transition metal dichalcogenide is selected from the groupconsisting of TiS₂, TiSe₂, MoS₂, MoSe₂, WS₂ and WSe₂.
 3. A methodaccording to claim 1, wherein the transition metal dichalcogenide isMoS₂.
 4. A method according to claim 1, wherein the transition metaldichalcogenide is in nanoparticle form, wherein the transition metaldichalcogenide nanoparticles have an average size between about 1 nm andabout 400 nm.
 5. A method according to claim 1, wherein the transitionmetal dichalcogenide is in nanoflake, nanosheet, or nanoribbon form,wherein the transition metal dichalcogenide nanoflakes, nanosheets, ornanoribbons have an average size between about 1 nm and about 400 nm. 6.A method according to claim 1, wherein the first electrolyte comprisesat least one helper catalyst.
 7. A method according claim 6, wherein thehelper catalyst is an imidazolium, pyridinium, pyrrolidinium,phosphonium, ammonium, choline, sulfonium, prolinate, or methioninatesalt.
 8. A method according to claim 6, wherein wherein the helpercatalyst is an imidazolium, pyridinium, pyrrolidinium, phosphonium,ammonium, choline or sulfonium salt having a counterion selected fromthe group consisting of C₁-C₅ alkylsulfate, tosylate, methanesulfonate,bis(trifluoromethylsulfonyl)imide, hexafluorophosphate,tetrafluoroborate, triflate, halide, carbamate, and sulfamate.
 9. Amethod according to claim 6, wherein in the first electrolyte the helpercatalyst is present in the aqueous solution in a concentration withinthe range of about 25 vol. % to about 75 vol. %.
 10. A method accordingto claim 1, wherein the first electrolyte is an aqueous solution.
 11. Amethod according to claim 1, wherein reducing carbon dioxide provides COor a mixture of CO and H₂.
 12. A method according to claim 1, whereinthe reduction of carbon dioxide is initiated at an overpotential of lessthan about 100 mV, and the reduction of the carbon dioxide has aFaradaic efficiency of at least 70%.
 13. A method according to claim 1,wherein the second electrolyte and the water comprise an aqueoussolution.
 14. A method according to claim 1, wherein the water oxidizingcatalyst comprises a cobalt-comprising film disposed on the anode.
 15. Amethod according to claim 1, wherein oxidizing water produces a mixtureof O₂ and H⁺.
 16. A method according to claim 1, wherein the firstcompartment is in ionic contact with the second compartment through aproton-conductive membrane.
 17. A method according to claim 1, whereinthe cathode and the anode are disposed on opposite surfaces of thephotovoltaic cell such that the photovoltaic cell is sandwiched betweenthe cathode and the anode.
 18. An electrochemical device having a firstand second compartment and at least one photovoltaic cell, wherein thefirst compartment includes a cathode in electrical contact with at leastone transition metal dichalcogenide, a first electrolyte, and carbondioxide, carbonic acid, or a carbonic acid salt; the second compartmentincludes an anode in electrical contact with at least one wateroxidizing catalyst, a second electrolyte, and water; the at least onephotovoltaic cell is in electrical contact with the anode and thecathode; and the first compartment is in ionic contact with the secondcompartment.
 19. A method of electrochemically reducing carbon dioxidein an electrochemical cell, comprising contacting the carbon dioxidewith at least one transition metal dichalcogenide in the electrochemicalcell and at least one helper catalyst and applying a potential to theelectrochemical cell, wherein the at least one transition metaldichalcogenide is MoSe₂, MoSe₂, WSe₂ or WS₂.
 20. A method ofelectrochemically reducing carbon dioxide according to claim 19comprising providing an electrochemical cell having a cathode in contactwith at least one transition metal dichalcogenide, and an electrolytecomprising at least one helper catalyst in contact with the cathode andthe at least one transition metal dichalcogenide, wherein the at leastone transition metal dichalcogenide is MoSe₂, MoSe₂, WSe₂ or WS₂;providing carbon dioxide to the electrochemical cell; and applying apotential to the electrochemical cell.